This invention relates to a semiconductor device, such as a vertical bi-polar transistor, which is incorporated in an LSI circuit or the like.
In the manner which will later be described more in detail, a conventional semiconductor device of the type described comprises a semiconductor substrate, a collector region of a first conductivity type formed on the semiconductor substrate, a base region of a second conductivity type reverse to the first conductivity type overlaid on the collector region, and an emitter region of the first conductivity type formed within the base region.
Such a semiconductor device is operated within a high electric current density range for the purpose of high speed operation. However, as will later be also described more in detail, a reduction of cut-off frequency is inevitable caused to occur due to Kirk effect, when the semiconductor device is operated within a high electric current density range.
Several solutions to the problem have been proposed, one of which is to increase a space charge density in a depletion layer between a base and a collector, and another one of which is to raise a speed of electrons running in the depletion layer.
For instance, an example of the former is described in Extended Abstracts of the 19th Conference on Solid State Devices and Materials, 1987, Tokyo, pp. 331-334, while an example of the latter is described in International Electron Devices Meeting, 1990, pp. 17.
However, no suggestion is made in the above-mentioned references about reducing breakdown voltage between the base and the collector due to an increase of the space charge density and the like.